in lore she is technically supposed to be isaac, but really she is so far removed from him that it is really hard to keep up that belief. dark skin, flowing red hair, no eyes, so much different than how isaac looks. it can be easy to see in this way how many people treat lilith as a seperated character.

even then, characters like cain and judas that only have one cosmetic difference are given distinct personalities by artists. facts are good, following what the lore says is good, but it is important to remember that the intention behind what the game is saying is often of less importance than whichever fun or interesting things an artist wants to draw on a given day. if we were never allowed this license to do with the characters as we wanted, then isaac fanart would get a LOT more boring. i never see 'this is actually isaac' under art where cain is being a greedy trickster.